The present invention relates to a speaker vertification system.
Systems in which a spoken password is compared with a password which has been registered in advance, to determine whether the speaker is the registered person, are called "speaker verification systems". It is thought that these systems will be useful as identification-verifying means in banks or check-in and check-out control systems, and practical realization thereof is expected.
A conventionally developed speaker verification system is practised in the following manner. In this system, the voice pattern of a passwork spoken by a registered speaker is stored as a reference pattern A and, during verification, an input password voice pattern X spoken by the speaker is compared with the reference pattern and the dissimilarity D(X, A) therebetween is calculated. If the dissimilarity is less than a predetermined threshold value .theta., it is recognized that the input voice pattern coincides with the reference pattern, and the speaker is judged to be the registered person. Conversely, if the dissimilarity is greater than the threshold value .theta., it is recognized that the two patterns do not coincide, and the speaker is judged to be an impostor.
The reliability of the prior art voice recognition system which has the function of verifying the speaker as described above is greatly influenced by the magnitude of the threshold value .theta.. There are two different kinds of erroneous operation of the speaker verification system, i.e., a false rejection of the true registered speaker as an impostor, and a false acceptance of an impostor as the true registered speaker. When the threshold value .theta. is reduced, the possibility of false rejections can be reduced, but the possibility of false acceptances is increased. Conversely, if the threshold value .theta. is increased, false rejections can be reduced, but false acceptances are increased.
When applied to person verification in banks or check-in and check-out systems, it is thought that the threshold value .theta. should be set to be rather low, in view of the seriousness of the result that might occur from a false acceptance. This inevitably leads to a system in which, when there is a false rejection, the speaker must pronounce the password repeatedly until it is correctly accepted. This system, however, leads to a reduction in service with speaker verification in banks, or to the inconvenience of blocked doorways with check-in and check-out systems. Moreover, it is difficult to set a suitable threshold value in this system. As a result, the following problems are caused. Even when the same person pronounces the predetermined password, the voice pattern changes each time, and the degree of change is different for each person. If the threshold value is set at a strict (or low) value, therefore, it often happens that a true registered speaker is recognized as an impostor. Conversely, of the threshold value is set at a rather loose (or high) level, it follows that an impostor could be erroneously recognized as a true registered speaker.
A system which has improved on these problems has been proposed by us, the inventors (in U.S. patent application Ser. No. 436,978). According to this proposed system, not only the reference pattern of a password, but also voice patterns which are similar to but different from the password are registered as control reference patterns. The speaker is recognized to be the registered person, and a coincidence signal is generated, only when the distance obtained as a result of matching with an input pattern satisfies the following conditions:
(a) The distance from the reference pattern is less than the distance from the control reference patterns; and PA1 (b) The distance from the reference pattern is less than a predetermined threshold value.
According to this system, because of judgement condition (a), the threshold value used in condition (b) can be loosely set. This provides the results that false recognitions of true registered speakers as impostors are reduced, and that false acceptances of impostors as true registered speakers can be prevented by condition (a).
This system, however, has the problem that control reference patterns must be registered by each person. Especially in the case of the general public use, this would increase false or erroneous operations due to the arbitrary selection and registration by each person of the control reference patterns.